As far as I understand lifetimes are already in the types, just elided most of the time. (That said, the type system is already full of marker traits, so keeping complexity from growing more and more is always an important goal. Hooweeever, this also means that people are pretty used to markers, so their individual cost is amortized over all of them basically. And adding one more might make sense, especially if it can be elided in general.)
